I recently had a sunroom floor remodel which included hydronic radiant floor heating set in gyp-crete, and topped with ceramic tile. The contractor/carpenter has done a number of excellent projects in my 1917 Arts&Crafts bungelow and has my complete trust. He tore out the old diagonal board subfloor, and instead of your standard plywood sheets over the joists, put in panels of plywood between the joists, level with their tops (very labor intensive). These were secured to 2x4 cleats with screws and construction adhesive. He did this so that we could get an extra thick layer of the gyp-crete. I think we got a good 3 inches. The tile guy did a great job, though it has only been a month and there are numerous fine cracks in the grout already! Now, the room (and floor) were ice cold when he did the tile work, and the HVAC guy did the final hook-up for the heat about a week later. The tile guy said he has never had this happen, and wasn't sure if it was from the sudden warming of the floor, or from movement in the subfloor. I can't imaging that thick layer of gyp-crete moving. Any thoughts? (the tile guy agreed to tear out the grout and re-do it free of charge, bless his soul).